What is a heart murmur and what causes it?

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What is a heart murmur and what causes it? Very often children are detected to have heart murmurs either during a routine visit to the doctor or from school health programs. This causes a lot of anxiety to the parents.
Normally heart produces two heart sounds, namely first and second heart sounds. An audible vibration in between the heart sounds is called a heart murmur.
They are generally of longer duration than heart sounds. Many murmurs are innocent murmurs due to enhanced flow across the valves producing turbulence. In children blood flow velocity is generally faster than in adults.
Normal laminar flow of blood across the valves do not produce any murmur. Murmurs are produced by turbulent flow, usually of higher velocity.
Some of the murmurs are due to abnormal blood flow as in the case of leaky valve, obstructed valve or abnormal communication between the chambers of the heart in the form of birth defects in its walls.
Innocent murmurs are more common in children than murmurs due to significant heart disease. Though innocent murmurs and murmurs due to heart disease can be often differentiated by physical examination, an ultrasound study (echocardiogram) is sometimes considered to rule out significant associated heart disease.
A common cause of murmur which is not due to heart disease is anemia. In anemia with associated low blood levels of hemoglobin, blood flow velocity is increased, causing turbulence and murmur.
Increased function of thyroid gland can also increase the blood flow velocity and cause murmur. This is associated with a fast heart rate as well, which is sometimes irregular.